At some point in their lives, many people may experience a hearing impairment, a full or partial decrease in their ability to detect or understand sounds. For many such hard of hearing individuals, the degree of hearing impairment varies by sound frequency. For example, many hard of hearing individuals may have little or no impairment at low sound frequencies, but varying degrees of impairment at higher frequencies. Loss of the ability to understand speech is generally regarded as one of the more detrimental aspects of hearing impairment. The frequency range from about 100 Hz-8 kHz is generally regarded as being useful for understanding speech.
In some cases, certain groups of hard of hearing individuals may share certain general characteristics. For example, statistical thresholds of hearing have been developed for men and women of various ages. However, most individuals have a distinct pattern of impairment that may vary from the statistical thresholds. Consequently, devices that are intended to compensate for an individual's personal hearing impairment often perform better when they are matched to the individual's distinct pattern of impairment.
Many hearing aids include several filters covering different parts of the audible frequency spectrum. By adjusting the response of the several filters, a hearing aid can often be “tuned” to compensate for an individual's distinct pattern of impairment.
At the present time, hearing aids are generally tuned by an auditory healthcare professional, often in a clinical setting. As part of the tuning process, an audiogram (a standardized plot representing the individual's hearing threshold) may be created, generally by performing a “pure tone audiometry” hearing test. Pure tone audiometry hearing tests usually involve presenting pure tones at varying frequencies and levels to an individual wearing calibrated headphones in a sound-controlled environment. The resulting audiogram may provide a starting point for tuning a hearing aid, but it is generally regarded that pure tone audiometry may not accurately measure an individual's perception of his or her hearing impairment. For example, pure tone audiometry may not be able to accurately measure the effect of “dead regions” in an individual's basilar membrane. In addition, pure tone audiometry may not measure various factors that are important to speech intelligibility.
Consequently, a further step in tuning a hearing aid generally includes asking the hearing aid wearer to subjectively evaluate speech. Often, the auditory healthcare professional will use his or her own voice as a test signal, speaking words or phrases and asking the hearing aid wearer to evaluate the spoken words or phrases. In many cases, the spoken words may include words selected from several pairs of words that differ only by an initial, final, or intervocalic consonant. The auditory healthcare professional may then use the individual's responses to adjust various hearing aid filter parameters.
However, this approach to speech intelligibility tuning may have drawbacks. For example, it may be difficult to achieve consistent results from tuning session to tuning session. In many cases, a hearing aid may need to be tuned multiple times, often over a period of days or weeks, before the wearer finds its performance acceptable. In many cases, the auditory healthcare professional's voice may change slightly or significantly from session to session (e.g., the professional's voice may be altered when he or she has a cold), so it may be difficult to compare results from session to session. In other cases, an auditory healthcare professional may retire or move, in which case, speech intelligibility may be evaluated based on a completely different voice.